1. Field of the Invention
The present invention relates to voltage regulator circuits. More particularly, the invention relates to multiple output switching mode power supply load over-current or short circuit protection.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode power converter provides a regulated DC output voltage to a load by selectively storing energy in an inductor coupled to the load by switching the flow of current into the inductor. There are several different topologies of switched mode power converter in common use, such as buck, boost, flyback, and forward converters, to name a few.
By way of example, a flyback converter is one type of switched mode converter that uses a power switch, typically a MOSFET transistor, to control the flow of current in the inductor. In order to obtain isolation between the input and output voltages, the flyback regulator may include a transformer that provides a physical dielectric barrier through which energy must pass prior to reaching the output. The power switch selectively couples the primary side of the transformer to the input voltage. The transformer may include plural secondary windings providing multiple positive or negative voltage outputs through respective rectifiers. During the on-state of the power switch, the full input voltage is placed across the primary winding of the transformer providing an increasing linear current ramp through the primary winding. When the power switch is turned off, the voltage across the power switch flies back to a voltage equal to the sum of the input voltage plus the turns ratio of the transformer multiplied by the output voltage (plus a diode voltage drop). During the flyback period (i.e., the power switch off-state), the output rectifier conducts, thereby passing the stored energy within the transformer core to the load. The flyback period continues until either the transformer core is depleted of energy, after which the voltage across the power switch returns to the input voltage, or the power switch is once again turned on.
A pulse width modulation (PWM) control circuit is used to control the gating of the power switch. In addition to controlling the power switch, the control circuit will also provide a stable reference voltage against which the output voltage may be compared to regulate operation of the power converter. The control circuit may also include a voltage error amplifier that performs the comparison of the reference to output voltage, and provides an error signal that determines the duty cycle of the signal used to control the power switch. It is also known to include a soft-start circuit that starts the power converter in a smooth fashion. The soft-start circuit will prevent the power switch from being driven to an on-state for a period of time sufficient to permit the input voltage to stabilize at a desired level. After this period of time is complete, the soft-start circuit applies power to the load in a controlled manner in which the duty cycle of the power switch is increased gradually. Without such a soft-start circuit, the voltage error amplifier would cause the duty cycle of the power switch to go to a maximum pulse width initially, causing an oscillating condition in which the current first overshoots and then undershoots before settling at a desired level.
Power converters generally include circuitry that protects the load in the event of a failure that occurs on the output lines of the power converter. Two types of failures that can occur on the output lines are over-current or short circuit. An over-current condition is typically caused by a short circuit in the load that is in series with a low load resistance. A short circuit is an over-current condition without the current limiting provided by any series resistance. In either condition, the power converter can deliver an excessive amount of current to the load, which could cause the rectifiers of the power converter to overheat and fail. To address these problems, protection circuits are used to sense a failure condition and reduce the power delivered to the load. These protection circuits operate in a cycle-by-cycle manner in which the duty cycle of the power switch is incrementally reduced with each successive cycle. A drawback of this operation is that the power converter is not completely shut down, and current continues to be delivered to the load.
Accordingly, it would be desirable to provide an improved way to protect a load of a multiple output switching mode power converter against an over-current or short circuit failure condition.
The present invention overcomes these drawbacks of the prior art by providing a protection circuit for a multiple output switching mode power converter that protects against an over-current or short circuit failure condition. The protection circuit activates the soft-start circuit of the PWM control circuit upon detection of the over-current or short circuit condition. The soft-start circuit then shuts off operation of the power converter and restarts the power converter after a period of time defined by the soft-start circuit.
More particularly, an exemplary switching mode power converter includes a transformer having a primary winding and a plurality of secondary windings. The secondary windings provide respective plural outputs through respective rectifiers. A switch is connected in series with the primary winding. An input voltage source is coupled to the primary winding. A controller is adapted to control operation of the switch to control a flow of current through the primary winding. The controller includes a feedback circuit to determine a duty cycle of the switch in response to a detected output voltage of at least one of the plural outputs and a detected current through the primary winding. The controller further includes a soft-start circuit used to inhibit activation of the switch for a predetermined period of time.
In an embodiment of the invention, the switching mode power converter further includes an over-current protection circuit coupled to the plural outputs so as to detect a change in either one of the plural outputs. Upon detection of the change, the over-current protection circuit activates the soft-start circuit to shut off delivery of the flow of current through the primary winding for the predetermined period of time. The soft-start circuit further includes a capacitor coupled to an internal voltage source of the controller, such that the period of time is defined by a charge time of the capacitor. The over-current protection circuit discharges the capacitor upon detection of the change in the load voltage. The over-current protection circuit further comprises a switch coupled to the capacitor, the switch connecting the capacitor to ground upon detection of the change in the load voltage. The over-current protection circuit further comprises an opto-isolator providing electrical isolation between the plural outputs and the controller.
In another embodiment of the invention, a method for protecting a multiple output switching mode power converter from an over-current or short circuit condition comprises the steps of: (a) detecting a change in one of the plural output voltages; (b) activating the soft-start circuit upon detection of the change in voltage; and (c) after a predetermined period of time, resuming normal operation of the power converter. The method further comprises isolating the plural outputs from the controller. The method further comprises maintaining the soft-start circuit in an activated state for a second predetermined period of time, wherein the resuming step occurs after completion of the second predetermined period of time.
A more complete understanding of the multiple output switching mode power supply having soft-start protection for load over-current or short circuit conditions will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.